In a number of surgical procedures referred to in the art as cardiopulmonary bypass operations, it is necessary to interrupt and suspend the normal functioning of the patient's heart and lungs and to temporarily replace the function of these organs with artificial blood handling and treating units in a lifesustaining extracorporeal blood flow circuit. In these procedures the main body of the patient's blood, which is called the venous return stream, is typically withdrawn from the patient through a venous cannula inserted into the right atrium, collected in a venous reservoir, and then passed through a blood pump (artificial heart), blood oxygenator (artificial lung) and arterial blood filter before being returned to the patient through an aortic cannula inserted into the aorta distal to the aortic arch. In conventional practice, the venous reservoir is a flexible transparent bag with a blood outlet in the bottom. Additionally, in typical practice, patient's blood from the surgical field, which is called cardiotomy blood, is gathered in one or more cardiac vacuum suckers and defoamed, filtered and collected in a cardiotomy reservoir and filter device. The treated cardiotomy blood is then conducted to the venous reservoir, where it is combined with the venous return blood. A highly effective cardiotomy reservoir and filter device is disclosed in the copending, commonly assigned, U.S. patent application Ser. No. 483,375, filed Apr. 8, 1983.
Typically, the volume flow rate of the venous return blood is at least three times that of the total cardiotomy blood. The cardiotomy blood can be quite "dirty", containing gas bubbles, fragments of tissue, bone chips, blood clots, surgical debris, etc. Thus, cardiotomy reservoir/filter devices usually include in the blood flow path a layer of a porous defoaming material and a layer of a depth filter material for filtering out particulate matter. By contrast, the venous return blood is a much cleaner stream.
The type of extracorporeal blood treatment circuit described above has been used for many years with great success in cardiopulmonary bypass and related surgical procedures. Nevertheless, improvements in circuit and equipment design are constantly being sought. In particular, it would be highly desirable to replace the venous reservoir and cardiotomy reservoir/filter units, two separate pieces of equipment, with a single unitary piece of equipment in order to reduce equipment costs and necessary inventory levels, reduce required priming volumes, simplify the steps of assembling, operating and disassembling the extracorporeal circuit, and reduce the possibilities of making incorrect connections between pieces of equipment. It is not, however, feasible to pass both the venous return blood and the cardiotomy blood through a cardiotomy reservoir/filter of conventional design along the same flow path, because excessive pressure drops and/or equipment sizes would have to be employed. Furthermore, it is not necessary to filter the relatively clean venous return blood through a depth filter material. In fact, such a filtering step should normally be avoided because it would subject the venous return blood to significant shear stresses, and thus some risk of damage to blood constituents, with little concomitant benefit.